Allow LogUnits to hold >1 word

LogUnits have been annotated with the autocorrected words, but
until now this was assumed to be a single word without spaces.
But spaceless typing can result in spaces in the LogUnit label.  With this
change, the LogUnit inspects the autocorrected text to determine how many
words were inserted, and counts them accurately.

This change corrects a privacy problem, which was that if the word sampling
algorithm chose a LogUnit that actually contained multiple words, then more
than two successive words would be included in the log.

Change-Id: I7c01c3dd3ac33d7e96c00836256bae9c14b124ed